1. Field of the Invention
The field of the present invention is internal combustion engines for motor vehicles and, in particular, utilization of the heat energy normally discarded in the exhaust of internal combustion engines by converting the heat to mechanical work in a highly efficient manner, thereby increasing the overall efficiency of fuel utilization.
2. Prior Art
The growing utilization of automobiles greatly adds to the atmospheric presence of various pollutants including oxides of nitrogen and greenhouse gases such as carbon dioxide.
Internal combustion engines create mechanical work from fuel energy by combustling the fuel over a thermodynamic cycle consisting typically of compression, ignition, expansion, and exhaust. Expansion is the process in which high pressures created by combustion are deployed against a piston, converting part of the released fuel energy to mechanical work. The efficiency of this process is determined in part by the thermodynamic efficiency of the cycle, which is determined in part by the final pressure and temperature to which the combusted mixture can be expanded while performing work on the moving piston.
Generally speaking, the lower the pressure and temperature reached at the end of the expansion stroke, the greater the amount of work that has been extracted. In conventional engine designs, expansion is limited by the fixed maximum volume of the cylinder, since there is only a finite volume available in which combusting gases may expand and still perform work on the piston. This makes it impractical to expand to anywhere near ambient temperature and pressure, and instead a large amount of energy remains and is normally discarded with the exhaust. The production of work from the initial expansion of combustion gases is commonly referred to as "topping," while the extraction of work from once-expanded gases is referred to as a "bottoming cycle."
Bottoming cycles are commonly employed as part of the combined cycle operation of steam power plants. "Performance Analysis of Gas Turbine Air-Bottoming Combined System," Energy Conversion Management, vol. 37, no. 4, pp. 399-403, 1996; and "Air Bottoming Cycle: Use of Gas Turbine Waste Heat for Power Generation," ASME Journal of Engineering for Gas Turbines and Power, vol. 118, pp. 359-368, April 1996 are representative of the state of the art in this field. Exhaust heat rejected from a primary gas turbine (the topping cycle) is used to heat water to produce steam that is expanded in a secondary steam turbine (the bottoming cycle) Although in this case the working fluid of the bottoming cycle is steam, other fluids having more favorable physical or thermodynamic properties may be used, for instance ammonia-water mixtures or even a gas.
Bottoming cycles that employ water/steam or any other recirculating medium as the working fluid must provide additional hardware for recirculation and purification. For instance, steam-based plants require a boiler, a sophisticated steam turbine, condenser, purification system to prevent mineral deposits and scaling, pumps, etc. For this reason, they are practically limited to stationary applications such as public power utilities and industrial plant use and are precluded from mobile applications such as motor vehicles.
Motor vehicles represent a large portion of total energy use in the world today. There are, of course, differences between stationary power plants and power plants of motor vehicles. First, motor vehicles usually do not employ a turbine in the topping phase and so produce a less uniform flow rate of gases in the exhaust. Second, for a motor vehicle the equipment devoted to the bottoming cycle should be low cost, relatively simple to operate and maintain, and lightweight. Third, in a motor vehicle the working fluid of the bottoming cycle should be safe and not require extensive recirculation hardware.
The use of air as a working fluid for stationary power generating applications has been investigated. In U.S. Pat. No. 4,751,814, "Air Cycle Thermodynamic Conversion System," a gas turbine topping cycle is combined with an air turbine bottoming cycle. Air is compressed in an intercooled multi-stage compression system that maintains air temperature as low as possible. Heat from the turbine exhaust is transferred to the compressed air via a counter flow heat exchanger, and the heated compressed air is expanded through an air turbine to provide at least sufficient work to run the compressors and preferably enough to use for other purposes. This system obviates sophisticated purification and processing of the working fluid (atmospheric air) if it is recirculated at all, and dispenses with bulky steam handling equipment. However, the system depends on turbine-based topping and bottoming apparatus which is not well suited to conventional motor vehicle applications.
Piston (or other means with sealed moving surfaces) compressors and expanders provide high efficiency for the processes of compression and expansion, but exhibit friction that is generally higher than a gas turbine of the same size (i.e., operating at similar gas flow rates). However, gas turbines (especially for the smaller sizes that would be needed for road vehicles) do not provide process efficiency as high as desired because of gas leakage around the edges of the turbine blades (the moving surfaces), which are not sealed.
Further, gas turbines operate at extremely high speed (often greater than 100,000 RPM), and the speed reduction gearing necessary to provide mechanical power at speeds usable in a mobile vehicle (e.g., less than 6,000 RPM) is costly and inefficient.